The invention relates to an apparatus for removing solid or liquid non-conductive particles from a gas stream wherein the gas stream is passed through a separating space bounded by two co-axial cylindrical electrodes and wherein an electric D.C. field is generated between the electrodes with such an intensity that substantially no ionization occurs in the gas stream.
An apparatus of this kind is indicated, in practice, as a dipole separator. Due to the use of a relatively low field intensity in the separating space, there is no, or substantially no ionization in the gas stream so that the separation is mainly based on the phenomenon that dipole charges are generated in the particles to be removed. In order to attain this effect, the field intensity in the separating space must be lower than a critical value which may amount to 25-50 kV/inch in the case that the gas stream consists of air.
The theory of dipole separation has been explained in an article by Dr. Gunter Zebel, entitled: "Ueber die Aggregatbildung zwischen kugelformigen Aerosolteilchen mit parallel ausgerichteten Dipolmomenten" in the German periodical "Staub", Vol. 23, Nr. 5, May 1963, pages 263-268. This article does not contain any information about the shape and the arrangement of the electrodes but it states very definitely that the force exerted on the particles to be removed is proportional to the gradient of the electric field intensity. In fact a dipole charge induced in a particle comprises a positive charge at one end and a negative charge at the other end; if the field is homogeneous in a direction perpendicular to the electrodes equal forces in opposite directions are exerted on both charges so that the particle has no inducement to move to either electrode. Thus in order to effect a separation the field must be inhomogeneous in a direction perpendicular to the electrodes.
A suitable construction for a dipole separator has been disclosed in U.S. Patent Application Ser. No. 447,775 (Van Diepenbroek et al.), filed on Mar. 6, 1974, now U.S. Pat. No. 3,970,437. In this construction the separating space is bounded by two co-axial cylinders. The inner cylinder is conductive whereas the outer cylinder is non-conductive and carries an electrode or an electrode system covering only a part of the surface of the outer cylinder, whereby the field is rendered inhomogeneous since it is locally concentrated at the outer cylinder.
The idea to use an electric field with a relatively low intensity so as to avoid any ionization has been disclosed in U.S. Pat. No. 1,992,974 in the name of Richard C. Thompson. In the apparatus described in this patent the electrodes consist of flat plates arranged in parallel whereby the field is homogeneous in a direction perpendicular to the electrodes and the above mentioned condition is not satisfied. The electrodes are all provided with a covering consisting of a non-conductive fibrous material intended to capture any conductive particles so that they are not repelled by the electrodes. Since the field is homogeneous there cannot occur a dipole separation in the normal sense of the expression. Presumably the operation is based on the capture of particles that move incidentally to the covering for instance by a turbulence in the gas stream.
U.S. Pat. No. 1,650,097 in the name of Walter A. Schmidt discloses a separator with co-axial cylindrical electrodes. However the field intensity is so large in this separator that there is a substantial ionization in the gas stream. The inner electrode is shaped as a thin rod in order to enhance the ionization. Further more the inner electrode is provided with a discontinuous covering consisting of a non-conductive material in order to provide for a uniform electric discharge.